result_try: relax ConvertWithTo constraint
result_try: relax ConvertWithTo constraint

Currently, the catch handlers in result_futurize_try are required to
return a future, although they are always being called with
seastar::futurize_invoke, so if their result is not future it could be
converted to one anyway. This commit relaxes the ConvertsWithTo
constraint in order to allow this conversion.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla